19. _Vowels and Diphthongs in unaccented Syllables._--The changes of the short vowels and of the diphthongs in unaccented syllables are too numerous and complex to be set forth here. Some took place under the first-syllable system of accent, some later (§§ 9, 10). Typical examples are _pep_E_rci_ from *_péparcai_ and _ónustus_ from *_ónostos_ (before two consonants); _concIno_ from *_cóncano_ and _hosp_I_t_I_s_ from *_hóstipotes_, _legImus_ beside Gr. [Greek: legomen] (before one consonant); _Sic_U_li_ from *_Siceloi_ (before a thick _l_, see § 17, 3); _dil_I_g_I_t_ from *_dísleget_ (contrast, however, the preservation of the second _e_ in _negl_E_g_I_t_); _occ_U_pat_ from *_opcapat_ (contrast _accipit_ with _i_ in the following syllable); the varying spelling in _monumentum_ and _monimentum_, _maxumus_ and _maximus_, points to an intermediate sound (_ü_) between _u_ and _i_ (cf. Quint. i. 4. 8, reading _optumum_ and _optimum_ [not _opimum_] with W. M. Lindsay, _Latin Language_ §§ 14, 16, seq.), which could not be correctly represented in spelling; this difference may, however, be due merely to the effect of differences in the neighbouring sounds, an effect greatly obscured by analogical influences.
Inscriptions of the 4th or 3rd century, B.C. which show original -_es_ and -_os_ in final syllables (e.g. _Veneres_, gen. sing., _navebos_ abl. pl.) compared with the usual forms in -_is_, -_us_ a century later, give us roughly the date of these changes. But final -_os_, -_om_, remained after -_u_- (and _v_) down to 50 B.C. as in _servos_.
20. Special mention should be made of the change of -_ri_- and -_ro_- to -_er_- (_incertus_ from *_encritos_; _ager_, _acer_ from *_agros_, *_acris_; the feminine _acris_ was restored in Latin (though not in North Oscan) by the analogy of other adjectives, like _tristis_, while the masculine _acer_ was protected by the parallel masculine forms of the -_o_- declension, like _tener_, _niger_ [from *_teneros_, *_nigros_]).
21. Long vowels generally remained unchanged, as in _compago_, _condono_.
22. Of the diphthongs, _ai_ and _oi_ both sank to _ei_, and with original _ei_ further to _i_, in unaccented syllables, as in _Achivi_ from Gr. [Greek: Achaiwoi], _oliivom_, earlier *_oleivom_ (borrowed into Gothic and there becoming _alev_) from Gr. [Greek: elaiwon]. This gives us interesting chronological data, since the _el_- must have changed to _ol_- (§ 16. 3) before the change of -_ai_- to -_ei_-, and that before the change of the accent from the first syllable to the penultimate (§ 9); and the borrowing took place after -_ai_- had become -_ei_-, but before -_eivom_ had become -_eum_, as it regularly did before the time of Plautus.